1. Field of the Invention
The present invention relates to a surface treated steel material having a layer of Zn- or Zn series alloy-layer and/or Zn composite material plated thereon. More particularly, the present invention relates to an electroplated steel sheet provided with the above mentioned layer for an anticorrosive protection of an automobile.
2. Description of the Related Arts
The de-icing rock salt sprayed in the winter on roads in arctic districts causes the corrosion of an automobile body, and the use of various plated steel sheets for an automobile body has been considered as an corrosion control measure. The corrosion of the outer surface of an automobile body is promoted by the gravel, sprayed rock salt and the like which are thrown up from the road surface and impinge on the automobiles traveling on a road (this impingement is referred to as chipping) at a speed equal to the running speed of the automobiles, i.e., from 50 to 150 km/hour. The resultant impingement force is very high and thus the paint coating on the automobile body may peel off, or flaws reaching the steel substrate may be formed. Water from melted snow or the salt in the de-icing material will seep into the peeled parts or flaws and exert a detrimental corrosive action on the steel substrate. Accordingly, Zn plated steel sheets have been employed as a corrosion control counter measure, since the Zn plating has a strong sacrificial anode effect and thus can satisfactorily protect the steel sheet from corrosion, even though the flaws reach the steel sheet. However, when a coating of paint is applied on the Zn plated layer, as in the case of an automobile outer panel, the paint bulging referred to as blistering sometimes forms in the paint coating.
The corrosive environments to which the steel sheets of an automobile are exposed make it necessary to provide uncoated steel sheets with a corrosion resistance, and in addition, a corrosion resistance after coating, including blister resistance, water adherence resistance, red rust resistance, and flow rust resistance.
These corrosion resistances, which are improved by the Zn plated steel sheets, are further improved by a Zn-series alloy layer, such as a plated layer of Zn-Ni, Zn-Fe, Zn-Co, Zn-Fe-Cr, Zn-Ni-Co, Zn-Cr, Zn-Mn, Zn-Ti, Zn-Sn, Zn-Cu, Zn-Cd, Zn-Pd, and the like, a multilayer plated steel having a plurality of layers of the above alloys (i.e., superimposed plated layers of different compositions), a steel sheet with a graded plating layer (i.e., composition of a plated layer varies along the thickness of the layer), and a steel sheet with a composite electroplated layer(s) (i.e., particles of oxide, such as SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, and the like, as well as metallic particles of Zn, Al, Cr, and the like are incorporated in the Zn plated layer. Regarding the composite electroplated layer, Japanese Unexamined Patent Publication No. 60-38480 discloses a Zn-oxide sol composite, Japanese Unexamined Patent Publication No. 60-141,898 discloses a Zn series alloy-oxide composite, and Japanese Unexamined Patent Publication No. 60-96786 discloses Zn-corrosion inhibiting pigments. The composite plating with nonmetallic particles is disclosed in Japanese Unexamined Patent Publication No. 61-64899. However, the particles precipitated and dispersed in the Zn or Zn alloy matrix are difficult to dissolve in the acidic solution and have a size which is not easily soluble.
The above described surface treated steel sheets have problems, as explained in detail below, in that, depending upon the intended environmental use of the automobiles, a further improvement in one or more of the corrosion resistances is needed.
A. The internal stress of the Zn series-alloy plated or Zn-composite plated steel sheets is higher than that of the ordinary electroplated steel sheet of Zn alone. The adherence of the former plated layer with the steel substrate (the adherence of the layer with the substrate, when in direct contact with the substrate, is hereinafter referred to as the "substrate adherence") is disadvantageously inferior to that of the latter plated layer.
B. Generally, the outer surface of an automobile body has a three-layer paint coating approximately 100 .mu.m in total thickness and comprised of the cationic ED paint, the intercoat, and the top coat. The shrinkage stress generated during the baking of these coats has an affect on the plated layers, in that the post-coat substrate adherence is inferior to the unpainted adherence.
C. Since the ambient temperature falls to around -50.degree. C., shrinkage of the paint coating occurs and multiplies the stress acting on the plated layer(s). This causes a reduction of the substrate adherence. Note, in the three layer-coating, there is a drastic lowering of the substrate adherence upon a fall in the temperature. When chipping occurs in a plated layer(s) exhibiting a lowered substrate adherence, the plated layer(s) of the Zn series-alloy plated or Zn-composite plated steel sheets is disadvantageously peeled off.
D. The adherence of plated layer(s) of the Zn series-alloy plated or Zn-composite plated steel sheets, in which the contact and noncontact of the plated layer(s) with the steel substrate is not taken into account, but instead, the adherence of plated layer(s) in the above steel sheets is generally considered, is hereinafter referred to as the "plating adherence". The plating adherence can be improved by interposing a covering layer consisting of one or more of Cr, Mn, Fe, Co, Ni, Cu, In, Zn, Cd, Sn, and Pb between the steel substrate and the Zn series-alloy plated layer, as disclosed in Japanese Unexamined Patent Publication No. 59-200789. In this publication, however, the plating adherence is verified under an extrusion formation (5 Erichsen bulging) of an unpainted article at an ordinary temperature, i.e., a mild condition. The interposed layer does not appear to satisfactorily improve the plating adherence under the severe conditions described in items B and/or C. Tetsu to Hagane (Bulletin of Iron and Steel Institute of Japan) 71 (1985), page 1273, discloses that a layer of one or more of Fe, Zn, Ni, Cu, and Sn interposed between the steel substrate and the Zn series-alloy plated layer provides a satisfactory plating adherence under the conditions of two coating layers, under the icing point, and in a Dupont impact test. However, an improvement in the plating adherence for a three layer coating appears to be unlikely.
E. The two-layer coating of Zn-Fe, Zn-Ni, and the like is one of the most effective methods for enhancing the water resistant adherence of the Zn series-alloy plated or Zn-composite plated steel sheets, while maintaining an excellent corrosion resistance thereof. In such a two-layer coated steel sheet, the lower plated layer consists of Zn-Fe, or Zn-Ni with a high Zn concentration, and the upper plated layer has a high Fe concentration. More specifically, Japanese Unexamined Patent Publication No. 58-58294 discloses a lower plated alloy layer consisting of Zn-Ni, and an upper plated layer of Fe or Fe-Zn alloy. Japanese Unexamined Patent Publication No. 59-89785 discloses a lower plated layer consisting of Zn, Zn-Ni alloy, or Fe-Zn alloy, and an upper plated alloy layer of Fe-Zn-Cr. Japanese Unexamined Patent Publication No. 60-131991 discloses a lower plated layer consisting of Zn or Zn alloy, and an upper plated alloy layer of Fe-P. Since in each of these disclosures the upper layer is a plated layer of Fe or Fe alloy, the phosphating film formed in the pretreating phosphating process prior to the cation ED coating can contain Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O (phosphophyllite) crystals in a large amount, thereby improving the poor water resistant adherence (adherence of paint coating after immersion in warm water) due to the lower Zn or Zn series-plated layer. When, however, a water resistant adherence as good as that of a cold rolled sheet is desired, at least 70% of Fe is necessary for the Fe concentration in the upper plated layer, according to research by the present inventors. On the other hand, an upper plated layer with such a high Fe concentration renders the unpainted corrosion resistance of the plated layers, as a whole, inferior to that of the lower single layer.
F. Further, an upper plated layer with such a high Fe concentration reduces the corrosion resistance after painting, so that red rust flows from the scribed parts, although such an upper plated layer is advantageous for lessening the occurrence of red rust. This appears to be attributable to a corrosion potential between the upper and lower layers so great that contact corrosion is liable to occur, and to an Fe concentration of the upper layer so high that red rusting occurs in the layer itself.